1. Field of the Invention
The present invention relates to a semiconductor pressure detecting device whose output voltage is non-linearly changed for the pressure change.
2. Description of the Prior Art
A semiconductor pressure detecting device, which transforms the distortion or the resistance change generated in a piezo resistance or the like in response to pressure into voltage to detect the pressure on the basis of the voltage, has been widely used in various fields, because it is very small sized, has a high precision in measurement and can be easily mass-produced (For example, see Japanese Laid-open Patent Publications No. 55-37906, No. 63-241439 and No. 9-61189.).
FIG. 5 is a circuit diagram showing an electrical construction of a conventional representative semiconductor pressure detecting device of the above-mentioned type. As shown in FIG. 5, the conventional semiconductor pressure detecting device 101 is provided with a sensor element section 102 having a Wheatstone bridge composed of four resistors R101 to R104 each of which is formed of a piezo resistance or the like, a differential amplifier 103 which amplifies the voltage outputted from the sensor element section 102, two operational amplifiers OP101, OP102 each of which amplifies the voltage outputted from the differential amplifier 103, and an output terminal 104 for outputting the amplified voltage.
In addition, the semiconductor pressure detecting device 101 is provided with another operational amplifier OP121 for generating reference voltage Vref for the differential amplifier 103 and operational amplifiers OP101, OP102, using driving voltage Vcc supplied from an external section. Hereupon, two resistors R111 and R112 are appended to the operational amplifier OP101 while four resistors R121 to R124 are appended to the operational amplifier OP102. Meanwhile, two resistors R105 and R106 are appended to the operational amplifier OP121.
Thus, in the conventional semiconductor pressure detecting device 101, the voltage corresponding to the pressure, which is outputted from the sensor element section 102, is amplified by the differential amplifier 103 and the operational amplifiers OP101, OP102, and then outputted from the output terminal 104 as the output voltage VOUT.
The conventional semiconductor pressure detecting device, for example shown in FIG. 5, has such an output property that the output voltage is linearly changed for the pressure change. In consequence, for example, in such a case that high precision is required in the low pressure region while high precision is not required so much in the high pressure region, the semiconductor pressure detecting device must have a design specification suitable for the precision in the low pressure region. On the other hand, if there exists a requirement contrary to the above-mentioned example, the semiconductor pressure detecting device must have a design specification suitable for the precision in the high pressure region. Therefore, there exists such a problem that the semiconductor pressure detecting device becomes expensive because it must be designed one by one for each of various specifications.
Meanwhile, in the conventional semiconductor pressure detecting device, the output voltage is generally inputted into an A-D converter in a micro computer or the like. In consequence, the semiconductor pressure detecting device is designed so as to have such an output property that the output is proportional to the change of the source voltage or the driving voltage (i.e. xe2x80x9csource voltage ratio propertyxe2x80x9d). On the other hand, the A-D converter also has an operating property proportional to the change of the source voltage. Therefore, if a common source is used for the both, there may be obtained such an advantage that errors of the both due to the change of the source voltage are canceled to each other. However, in that case, because the source voltage ratio property can not be obtained when it is constructed as a non-linear circuit, there may occur such a problem that the errors in measurement become larger.
Moreover, in the conventional semiconductor pressure detecting device, if it is constructed as a non-linear circuit, there may occur such a problem that errors occur due to the offset voltage (i.e. output voltage when the pressure is 0) when the output voltage is fed back. For example, the output voltage is required to have a non-liner property which merely corresponds to the pressure change only. However, when the offset voltage of the semiconductor pressure detecting device is not 0V, there may occur such a problem that a non-linear correction is performed due to the offset voltage even if the pressure change is 0.
Further, in the conventional semiconductor pressure detecting device, because the non-linear property can not be adjusted, there may occur such a problem that errors occur due to dispersion among the products such as the non-linearity of the sensor element (due to the balloon effect etc.) or the non-linearity of the IC circuit (for example, due to the voltage dependency of the resistor).
The present invention, which has been developed to solve the conventional problems described above, has an object to provide a semiconductor pressure detecting device having the following features. That is, a non-linear output property can be obtained. A source voltage ratio property can be obtained. Errors due to the offset voltage can be diminished. In addition, the non-linear property can be easily adjusted.
A semiconductor pressure detecting device according to the present invention, which has been developed to solve the above-mentioned problems, includes a sensor element section, a voltage amplifying section and an output voltage feedback section. The sensor element section has semiconductor resistors (for example, piezo resistors), each of which is distorted in response to pressure to change its resistance in response to a distortion thereof. The sensor element section is supplied with a reference source voltage Vsen while outputting a voltage corresponding to the pressure. The voltage amplifying section amplifies the voltage outputted from the sensor element section to output the amplified voltage as an output voltage VOUT. The output voltage feedback section feeds back the output voltage VOUT of the voltage amplifying section to the reference source voltage Vsen so as to impart a non-linear property for the pressure to the output voltage.
According to the semiconductor pressure detecting device, a non-linear output property of output-attenuating type or output-increasing type can be easily obtained because the non-linear property for the pressure is imparted to the output voltage by feeding back the output voltage of the voltage amplifying section to the reference source voltage.
In each of the above-mentioned semiconductor pressure detecting devices, it is preferable that the feedback ratio of the output voltage VOUT of the voltage amplifying section for the reference source voltage Vsen can be set to an arbitrary value. In this case, the non-linear property of the device can be easily adjusted.
In each of the above-mentioned semiconductor pressure detecting devices, the output voltage feedback section may impart the output voltage VOUT with such a non-linear property that the higher the output voltage VOUT of the voltage amplifying section becomes, the higher the reference source voltage Vsen becomes. In this case, there may be obtained such a non-linear output property of output-increasing type that the sensitivity of the device is raised in the high pressure region while the sensitivity is lowered in the low pressure region.
Alternatively, the output voltage feedback section may impart the output voltage VOUT with such a non-linear property that the higher the output voltage VOUT of the voltage amplifying section becomes, the lower the reference source voltage Vsen becomes. In this case, there may be obtained such a non-linear output property of output-attenuating type that the sensitivity of the device is raised in the low pressure region while the sensitivity is lowered in the high pressure region.